This application claims the priority of German Application No. 101 11 560.1-43 filed Mar. 10, 2001, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and a device for removing carbon monoxide from a gas stream.
In fuel cells, hydrogen and oxygen react to form water while yielding an electrical current. The efficiency of the fuel cell depends, among other things, on the purity of the reactants. The production of hydrogen, e.g., for PEM fuel cell systems, from carbon-containing substances, such as methanol, benzine, naphtha, methane, etc., requires some sort of reforming in a gas production system. This may be steam reforming, partial oxidation or autothermal reforming. In these processes, in which hydrogen is produced from a liquid or a gaseous fuel by reforming, the fuel stream does not consist of pure hydrogen but includes other, partially undesirable, components. Particularly the carbon monoxide (CO) that is produced in this process presents a problem, since it is harmful to many catalystsxe2x80x94not just in fuel cellsxe2x80x94and, furthermore, may not be emitted into the environment. Its proportion in the reformate stream must consequently be reduced to a range clearly below 100 ppm.
There are various known techniques to reduce undesirable components. Membrane processes use membranes that are more or less permeable to hydrogen to eliminate impurities. One hundred percent selectivity to hydrogen is offered only by palladium-containing membranes, which today are still very expensive and difficult to process. Other materials allow not only hydrogen to pass through but also undesirable impurities, so that additional purification steps are required. All these types require a pressure drop as the driving force for permeability, which means an increased expenditure of energy.
Shift reactors use the water-gas shift equilibrium to reduce the CO content with the addition of water. A temperature dependent reaction equilibrium is established, in which CO and water form CO2 and hydrogen and, conversely, CO2 and hydrogen form CO and water.
The CO content can also be reduced by selective oxidation of the CO with the addition of air, whereby CO2 is formed.
Both of these processes are well suited for primary purification, since they can lower the carbon monoxide content to approximately 0.1 vol.xe2x88x92% at relatively low cost. Thereafter, for a further reduction to the desired concentration, the temperature of the reaction must be reduced for reasons of equilibrium. Both shift reaction and selective oxidation, however, are slow at the required low temperatures. As a consequence, they require an increased amount of catalysts resulting in either large reactors (shift stages) or simply higher costs due to an increased precious metal content (CO oxidation). Furthermore, for full function, the catalysts must be brought to a defined operating temperature. In the case of selective oxidation, air must be added in metered quantities, which entails costly equipment. The introduced air, due to the nitrogen contained therein, moreover dilutes the hydrogen stream, which is undesirable under certain circumstances.
Primary purification is possible without great complexity, e.g., by means of membranes with low selectivities, selective oxidation stages, or shift reactors. In contrast, eliminating the approximately 0.1 vol.xe2x88x92% CO residue remaining in the reformate stream after primary purification poses greater challenges.
WO 00/16880 A1 discloses a method which removes CO from a hydrogen rich fuel by means of a catalytic material that preferentially adsorbs CO. The catalytic material is regenerated by an oxidizing agent, which reacts with the adsorbed CO. The reaction is initiated by an electrical current and makes use of the CO property whereby it is capable of quickly and completely covering the surface of many substances. This is one reason for its toxicity and its harmful influence on PEM fuel cells. Subsequently, the adsorbed amount of CO is electrochemically oxidized. The CO2 thus formed does not continue to adhere to the surface, which is free to be recoated. For electrochemical oxidation, a voltage is applied between a working electrode and a counter electrode, which are separated by an ion conducting membrane, so that an electrical current can flow across the membrane. This causes problems, however, if the degree of CO coverage is too high.
Thus, the object of the invention is to provide a method and a device for removing carbon monoxide from a gas stream, which obviates these problems.
The invention provides an electrochemical cell in which a membrane-electrode unit is configured so way that a reference electrode, which is independent of the current flow, is provided in a counter electrode, in order for the oxidation conditions for CO removal to be detected and/or adjusted independently from the coating state of the working electrode.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.